The present invention relates to an anti-lock brake system having traction slip control which includes a pedal-actuated, preferably auxiliary-force-assisted braking pressure generator having a master cylinder, to which the wheel brakes are connected by way of main brake lines, an auxiliary-pressure hydraulic supply system and wheel sensors and electronic circuits for determining the wheel rotational behavior and for generating electric braking-pressure control signals which, for the purpose of slip control, serve to control electromagnetically actuatable pressure-fluid inlet valves and outlet valves provided in the pressure-fluid lines. The brake lines communicate by way of supply lines with electrically driven pumps whose suction ports are connected to the supply reservoir by a suction line.
In known brake systems of this type, such as the German published patent application Nos. 30 40 561 (See U.S. Pat. No. 4,415,210) and 30 40 562 (See U.S. Pat. No. 4,416,491), a master cylinder having a hydraulic brake power booster connected upstream thereof is used as a braking pressure generator. The auxiliary-pressure supply system includes a hydraulic pump and a hydraulic accumulator, from which auxiliary pressure proportional to pedal force is delivered on brake application with the aid of a control valve. On the one hand, this dynamic pressure is transmitted by way of the master cylinder into the static brake circuits connected to the master cylinder. On the other hand, the wheel brakes of one axle, preferably those of the rear axle, are in direct communication with the pressure chamber of the booster into which the pressure proportional to pedal force is introduced through the control valve. For the purpose of slip control, inlet valves are provided in both the static circuits and the dynamic circuit, which valves normally assume their opened position and which, in the event of an imminent locked condition of a wheel, serve to shut off the pressure-fluid flow to the wheel brake effected.
There are also provided outlet valves which allow pressure fluid to discharge from the effected wheel brake to the pressure-compensating reservoir. On commencement of slip control, the booster chamber into which the controlled pressure from the auxiliary-pressure supply system is introduced, in connected by way of a main valve with the static brake circuits of the master cylinder in order to replenish the quantity of pressure fluid which is removed from the static circuit through the outlet valves. In addition, for safety reasons, the piston of the master cylinder, or pistons in the case of a tandem master cylinder, are reset or fixed by means of a positioning device. The structural complexity required for generating, storing and controlling the hyraulic auxiliary pressure, for dynamic fluid delivery into the static circuits and for safeguarding the brake functions on failure of individual circuits is considerable.
In brake systems of this type, the control signals for the inlet valves and outlet valves are generated by means of electronic circuits, the inputs of which are connected to wheel sensors, for example, inductive pickups, for measuring wheel rotational data, and which are able to react to changes in the wheel rotational behavior indicative of an imminent locked condition. The control signals operate the valves to maintain the pressure at the wheel concerned constant, by reducing and increasing the pressure required.